A left-handed block spiral is created by the following pattern: Start with a 1x1 square [ a(1)=1 ]. Attach a block of length 2 and width 1 (a horizontal 1x2 rectangle) to the upper side of the square [ a(2)=2 ] pointing to the right. Attach to the corner a vertical 2x1 block (rectangle). This block has length 2, so a(3)=2. Continue with a horizontal 1x2 block, so a(4) = 2. And so on. See the sketch shown in the link.